The present invention relates to a saw blades and, in particular, to a method and apparatus for strengthening and leveling circular saw blades.
It is very important that a saw blade be designed not to deform when operating under normal conditions. For a circular saw blade normal operating conditions involve high circular speeds and high loads imposed during cutting. When a circular saw operates at the high rate of rotation necessary to cut wood, metal and other materials, there is a tendency for the outer edges of conventional blades to deviate from the cutting line, either to one side or the other, or to both sides, producing a wavy cut. The wobbling of the cutting edge can also urge the saw edge to continue its cut to one side or the other of the intended line. The nature of the circular saw then tends to continue to lead the blade in that direction.
This uneven cutting is a result of invisible defects in the saw blade, such as minute surface deformities and improper distribution of residual tension stresses in the blade. The centrifugal force, which results from high blade rotational speeds, induces substantial tension stress in the blade. This tension is superimposed on any residual stresses in the blade, such that the resulting non-uniformity of tension produces uneven strain. Any irregularities in the blade surface also interfere with true running, as the tension changes when the blade approaches its cutting speed. Thus, a blade may be weakened or deflected from a true course by either of these types of defects.
Areas on the surface of a blade wherein residual tension stresses vary significantly from the normal tension level commonly referred to as xe2x80x9ctightxe2x80x9d or xe2x80x9cloosexe2x80x9d spots. Surface deformities include convex and concave deviations from a flat plane, including bulges, ridges, kinks, etc. Consequently, a critical part of the maintenance of circular saws is removing these dents or bends, i.e., leveling. Also, it is quite common to tune the saw blade to operate at typical rotational speeds by stretching the blade in specifically determined locations to cause tension at the periphery of the saw plate, i.e., tensioning. Leveling and tensioning operations have typically been done manually using various devices, such as hammers, anvils and gauges. Several equipment manufacturers build xe2x80x9cstretcher rollsxe2x80x9d which assist in such manual tensioning and leveling of saw blades. A typical circular saw blade is stamped from sheet metal with a punch press. This sheet metal is usually low-carbon steel with a Rockwell hardness rating of approximately Rc 22 (diamonds have an Rc rating of about 100). In the cutting tool industry, materials with an Rc rating of 22 are considered relatively soft, generally acceptable for consumer-grade blades but not rigid enough for professional use.
Several solutions for creating harder and more level saw blades have developed in the art. One solution is to harden the blade after it is cut. Another is to punch press the blade from the hardest steel the dies can handle. The capabilities of the punch limit the type and hardness of the steel that can be used. Either way, the maximum hardness of a low-carbon blade will be around Rc 29.
An alternate means for hardening a circular saw blade is to use a high carbon steel that generally has a hardness of around Rc 40-45. However, the hardness of this material inhibits the use of a punch press. Instead, alternate cutting procedures must be used, such as through the use of lasers. Laser-cut blades cost more than die-cut blades, due in part to the increased time involved in cutting the blade and the need for more expensive laser cutting machinery. On the other hand, a laser makes it easier (and cheaper) to produce small runs of specialized blades because a manufacturer does not have to purchase expensive dies.
Another conventional approach to hardening a circular saw blade is by tensioning. The plate of a top-quality blade is tensioned to increase its strength and prevent it from vibrating at high speeds. The idea behind tensioning is that the metal is compressed in one part of the plate. The compression dampens the blade""s natural tendency to vibrate at high speed, essentially changing the natural frequency of the material. Since the outer edges of blades can spin as fast as 12,000 surface feet per minute, the goal is to stiffen the blade such that it does not vibrate at this anticipated speed. Tensioning is an important feature for blades intended to produce fine-finish cuts. Tensioning is especially important for thin blades which are more prone to bending and vibrating during cutting.
One method used in the past by manufacturers to tension blades was by carefully striking certain parts of the plate with a hammer. The sawsmith was the skilled toolmaker who performed this manual tensioning. Due to the high precision necessary and time consuming labor involved, the blades produced by a sawsmith were more expensive than their mass produced counterparts. Today, most toolmakers tension a blade by running the rim between a pair of wheels as shown in RE 36,409 to Brown. This is known as rim tensioning.
FIG. 1 illustrates a convention rim tensioning system As shown, wheels W are used to stiffen the circumferential rim surface by squeezing a small segment of the blade B as the blade rotates on shafts S. One problem with conventional tensioning is that, as the wheels turn and compress the material, they produce heat in the blade that causes the material to crawl out of true level (i.e., out of plane). At best, conventional rim tensioning systems using wheels achieve only about a 0.004 inch planar tolerance of the blades. Another drawback of conventional rim tensioning machines is that because of the size of the wheels, the tensioning is typically performed two inches from the outer edge and is located at a set radial position on the blade.
A need, therefore, exists for a device that provides quick and accurate tensioning and leveling of a saw blade.
According to the present invention there is provided an apparatus for producing a harder and more level saw blade that has a planar tolerance down to 0.002 inches, and for providing increased tolerance using a less expensive machine than conventional rim tensioning machines. Instead of the wheels of the prior art, the present invention uses bearings, preferably spherical rollers, attached to pivotally mounted movable arms. The rollers are radially offset from each other along the same radius of a saw blade on opposing faces of the blade. The rollers generally have a diameter of between approximately xc2xd inch to about 3 inches. The preferred rollers have a diameter of about 1xc2xc inches and apply approximately 1200 pounds of point pressure to the face of the blade. In one embodiment, the rollers are spherical hardened steel with chrome plating. Any roller that is capable of bending steel could potentially be used in the present invention provided it can apply sufficient point pressure to create the desired effect.
Due to their smaller contact area on the blade, the spherical rollers create less friction and consequently generate less heat on the blade. Generally greater heat curls a blade out of plane, but the rollers of the invention generate less friction and less heat, and hence less curling than is found in the prior art.
In one embodiment, the rollers can be moved radially along the surface of the blade, which allows leveling of the blade between its center and edge. This adaptability of the invention allows for leveling and strengthening of different sizes of blades.
A method for producing the stronger and more level saw blade is also disclosed.
The foregoing and other features of the invention and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive.